Solid precursor feed system for thin film depositions

ABSTRACT

A dry powder MOCVD vapor source system is disclosed that utilizes a gravimetric powder feeder, a feed rate measurement and feeder control system, an evaporator and a load lock system for continuous operation for thin film production, particularly of REBCO type high temperature superconductor (HTS) tapes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/756,463 filed Apr. 15, 2020, which is a national stage application ofPCT application No. PCT/US19/68194 which claims priority and benefitfrom U.S. Provisional Patent Application No. 62/817,909 filed on Mar.13, 2019. The entire contents of each application listed above are ofwhich is incorporated in their entirety herein by reference.

BACKGROUND OF THE INVENTION Technical Field

Embodiments of the subject matter disclosed herein generally relate tosystems and methods for supplying powdered materials to a vapordeposition reactor and more particularly for fabricatinghigh-temperature superconductors from solid precursor materials.

Discussion of the Background

In vapor deposition processing, particularly high temperaturesuperconductor fabrication via MOCVD processing, there is a need toaccurately deliver to the reactor deposition chamber precise and tightlycontrolled quantities of precursor materials. Most common MOCVDprecursors for oxide superconductor deposition are solid at roomtemperature with low vapor pressures and require elevated temperaturesin order to achieve evaporation typically in the range of approximately150-300 C. Early approaches for generating precursor vapor utilizedbubblers with dissolved or suspended solid precursors but were difficultto use and required exposure to high temperatures for an extended periodof time, which caused degradation of the compounds and was plagued byhighly variable and unpredictable vapor delivery rates.

Oxide superconductor MOCVD processing using precursor vapors generatedby the flash evaporation approach began in the early 1990s. When flashevaporated, the bulk of precursor material is kept at room temperature,which preserves its properties, and small portions of the bulk materialare sequentially evaporated. Historically, the first implementation ofthe flash evaporation approach was the aerosol MOCVD vapor source[Langlet 1989]. One or several precursor powders were dissolved in anorganic solvent and this solution was nebulized and fed into a heatedevaporator in aerosol form. This solution-based approach wassubsequently improved by directly injecting liquid solution into theevaporator [Felten 1995]. This technique is currently used commerciallyfor second generation (2G) high-temperature superconductor (HTS) wireproduction but suffers from a number of drawbacks. Firstly, condensationof precursor within the evaporation and delivery system can lead tofouling, plugging and variable delivery to the reaction zone, as well asloss of expensive precursor material. To obviate these issues, theentire CVD delivery system must typically be insulated and heated withheat tape or other means. Further, large quantities of solvent vaporgenerated along with the precursor vapor is known to be a potentialproblem in the deposition process, particularly for REBCO (to bediscussed later) type superconductors.

Therefore, several implementations of solvent free dry MOCVD vaporsources were developed over the years: band flash evaporation sources[Kaul 1993; Klippe 1995]; solid source based on a vibratory feeder[Samoylenkov 1996]; solid source based on a grinder feeder [Hubert et.al., U.S. Pat. No. 5,820,678]; and solid source based on a volumetricfeed screw type feeder [Eils 2011]; while others developed approaches tomechanically meter the powder feed, [e.g. Long et. al., U.S. Pat. No.8,101,235] to further improve the performance of a volumetric typefeeder. These solid source approaches have their own set of drawbacks.For example, self-segregation of particles and vortexing of powderwithin the powder hopper and other detractions can cause highly variablepowder delivery rates that are difficult to control and predict.

High temperature superconductor thin film texture, growth rate, andfinal conductor performance characteristics are particularly sensitiveto factors related to the precursor delivery system. Materials havingsuperconducting properties at liquid nitrogen temperature (77K) includeYBa₂Cu₃O_(7−x) (YBCO) as one of a group of oxide-based superconductorscalled high temperature superconductors (HTS). High temperaturesuperconductors provide the potential for development of superconductorcomponents at higher operating temperatures compared to traditionalsuperconductors that operate at liquid helium temperature (4.2K).Superconductors operating at the higher temperatures enable the abilityto develop superconducting components and products more economically.After the initial discovery of YBCO superconductors, othersuperconductors were discovered having a similar chemical compositionbut with Y replaced by other rare earth (RE) elements. This family ofsuperconductors is often denoted as REBCO where RE may include Y, La,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu.

There are several methods for the deposition of REBCO type thin films inthe manufacturing of REBCO wire including metal organic chemical vapordeposition (MOCVD), pulsed laser deposition (PLD), reactiveco-evaporation (RCE), and metal organic deposition (MOD). Many methodswithin the category of physical vapor deposition (PVD) techniques sufferfrom generally low growth rates; a requirement for high vacuum; need forcontinual source change-out; moderate area coverage and a restriction toonly line-of-sight deposition. Such limitations, especially the lowgrowth rates, are problematic for the economically viablecommercialization of the YBCO film technology for HTS wires and tapes.MOCVD with highly controlled precursor delivery can overcome many ofthese drawbacks and produce high quality thick superconducting YBCO thinfilms for coated conductor applications.

MOCVD technology has been directly applied to YBCO film growth and hasshown the capability for fabrication of high quality YBCO throughmodification of traditional semiconductor MOCVD for higher temperatures,oxidizing atmospheres and lower vapor pressure precursors (Zhang et.al.). The higher temperatures (more than 200K higher than that used forsemiconductor III-V compound MOCVD) require improved reactor designs andimproved heaters, and the lower vapor pressure precursors requireenhanced attention to precursor vapor flow control and stability. Theinitial results were promising, and for YBCO films grown on singlecrystal oxide substrates Tc>90K and Jc>10⁶ A/cm² were realized (Schulteet al.).

With the discovery of high temperature superconductor (HTS) materials;one of the foci was directed towards the development of HTS wire forhigh-power electrical applications. Such applications include, but arenot limited to, transmission cables, distribution cables, electricmotors, electric generators, electric magnets, fault current limiters,transformers, and energy storage. For the HTS wire to be a successfulsolution for these high-power electrical applications, it needs to meetthe high-power electrical requirements of the different applicationswhile being low enough in cost to meet the commercial requirements forthese applications.

One of the primary electrical characteristics of interest is thecritical current of the HTS wire. The critical current (Ic) is theelectrical current at which the superconductor loses its superconductingproperties and becomes non-superconducting. The critical current of thesuperconductor is affected by the temperatures and magnetic fieldsexperienced by the superconductor. The higher the temperature andmagnetic field, the lower the critical current. To be able to meet thetechnical requirements for the variety of applications, the HTS wireswill need to have high enough critical currents in the temperatures andmagnetic fields experienced by these applications.

One of the key approaches for increasing the critical current carryingcapacity of the superconductor is through the introduction of magneticflux pinning material into the superconductor. At higher magneticfields, type II superconductors allow magnetic flux to enter inquantized packets surrounded by a superconducting current vortex. Thesesites of penetration are known as flux tubes. Flux pinning is thephenomenon where free motion of magnetic flux tubes in type IIsuperconductors is inhibited due to their interaction with defects inthe superconducting material. A flux tube which is adjacent orencompassing such a defect has its energy altered and its motion throughsuperconducting material is impeded. Flux pinning seeks to takeadvantage of the dual critical fields that allow penetration of magneticfield lines into type II superconductors and which limit performancecharacteristics. Increased anisotropy and reduced current carryingcapacity results from unpinned flux tubes which aid the permeation ofmagnetic flux. Flux pinning is thus desirable in high-temperaturesuperconductors to prevent “flux creep”, which induces voltage andeffective resistance of the conductor and diminishes critical current(Ic) and critical current density (Jc).

Thus, the inclusion of pinning sites or centers that act as magneticflux pinning centers within the superconductor aid in the improvement ofcritical current carrying capacity. The pinning centers may be composedof specific compositions of non-superconducting material with specificorientations. Such centers may generally be referred to as pinning sitesor centers, flux pinning centers, defects, or defect centers. Thepresence of these flux pinning centers provides the wire the ability toimprove critical currents, even in high magnetic fields.

As with any superconducting wire, one of the key objectives has been toimprove the flux pinning properties and in turn, improve the Ic of theREBCO wires. Many processes have been investigated to produce a REBCOsuperconductor film with nanoparticle inclusions as pinning centers tofurther improve current capacity. The REBCO fabrication process has beenvaried to naturally create non-superconducting impurities such as Y₂O₃and Y₂BaCuO₅ in specific orientations relative to the superconductinglayer to yield improvements in flux pinning and corresponding Ic.

Other materials that are not part of the REBCO group of elements areknown to be introduced into the superconductor layer to create thenon-superconducting particles. Materials such as BaMO₃ where M may beTi, Zr, Al, Hf, Ir, Sn, Nb, Mo, Ta, Ce, V are added as doping materialto create the non-superconducting nanoparticles.

This doping of foreign material combined with the columnar distributionof preferentially c-axis orientated nanoparticles has yielded REBCO wirewith improved performance and increased Ic, especially in high magneticfields, as compared to non-doped material. However, the productionmethods to produce these nanodots and nanorods are highly complex inorder to deposit the doped material in specific super structures (e.g.columns) and orientations relative to the superconductor layer.Difficulties in achieving correct preferential orientation of the dopingmaterial restricts the growth rate of the wire which adds productiontime, and concomitant cost and complexity.

Thus, it is of great value to develop a superconducting articlefabrication process with precise and highly controllable precursordelivery system to produce high performance HTS wire that meets the Icrequirements of high-power applications, even at high magnetic fields.It is a further objective to produce a superconductor capable of meetingthose requirements at a high growth rate to enable production withcommercially attractive economics. Hence, reducing the variability ofprecursor delivery to the deposition zone that is inherent in currentprecursor delivery technologies has the potential to achieve high growthrates whilst maintaining desired crystallographic structure and pinningcenter distribution within the thin film for optimum current carryingcapacity even in high magnetic fields.

SUMMARY OF EXAMPLE EMBODIMENTS

According to an embodiment, there is a precursor feed system fordeposition of thin films. The system includes a powder feeder assemblywith a load lock assembly; a weighing mechanism configured to providecontinuous mass data of precursor powder in the powder vessel; a controlsystem; and an evaporator. The control system data processor convertsthe continuous mass data from the weighing mechanism to a feed screwrate to deliver a target precursor powder feed rate to the evaporator.

According to another embodiment, there is a precursor feed system fordeposition of thin films. The system includes a powder feeder assembly;a load lock assembly; a control system; and an evaporator. The controlsystem data processor converts a process variable input to a feed screwrate to deliver a target precursor powder feed rate to the evaporator.

According to yet another embodiment, there is a method for manufacturinga high temperature superconductor. The method includes introducing asubstrate to a reactor; providing an evaporator coupled to a precursorpowder feed assembly having a screw feed device and a precursor powdervessel coupled to a weighing mechanism; loading a precursor powder to aload lock assembly, wherein the precursor powder is comprised of atleast one component of a high temperature thin film superconductor;monitoring the precursor powder vessel weight; controlling the feedscrew rate based upon the powder vessel weight in order to provide atarget precursor powder feed rate to the evaporator; evaporating theprecursor powder in the evaporator; transporting the evaporatedprecursor into the reactor; and depositing the thin film upon thesubstrate in the reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 shows an exemplary powder feed system.

FIG. 2 shows an exemplary powder feed assembly.

FIG. 3 shows an exemplary load lock assembly.

FIG. 4 shows an exemplary control system.

FIG. 5 shows an exemplary plot of the performance of the actual feedrate vs. time for a given target.

FIG. 6 shows an exemplary evaporator assembly.

FIG. 7 shows an exemplary powder feed and reactor system.

FIG. 8 shows an exemplary architecture of a high-temperaturesuperconductor.

FIG. 9 shows an exemplary PAMOCVD reactor.

FIG. 10 shows an exemplary method for manufacturing a high-temperaturesuperconductor.

FIG. 11 shows an exemplary plot of the HTS performance vs. positionoverlaid with powder feed weight control variable.

DETAILED DESCRIPTION OF EXAMPLES OF THE INVENTION

The following description of the embodiments refers to the accompanyingdrawings. The same reference numbers in different drawings identify thesame or similar elements. The following detailed description does notlimit the invention. Instead, the scope of the invention is defined bythe appended claims. The following embodiments are discussed, forsimplicity, with regard to a system for precisely controlled solidprecursor delivery for deposition of thin films, particularlysuperconductor tapes. However, the embodiments discussed herein are notlimited to such elements.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the subject matter disclosed. Thus, the appearance of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout the specification is not necessarily referring to the sameembodiment. Further, the described features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Many epitaxial growth systems are known in the art to require precisefeed of precursor reactants to a vapor deposition reactor includinghigh-temperature superconductors (HTS). Embodiments of the presentinvention include a precursor feed system capable of preciselycontrolled delivery of solid phase precursors to a vapor depositionreactor suitable for HTS fabrication and other deposited thin filmapplications.

The main components of an exemplary embodiment of the powder precursorfeed system are illustrated in FIG. 1. Main feed system componentsinclude a powder feeder assembly 100 comprising a feed screw device 220and a powder vessel 215 for housing a precursor powder 212; a load lockassembly 110 comprising a pressure isolated chamber for reloading thepowder feeder assembly powder vessel 215; a weighing mechanism 120coupled to the powder vessel and configured to provide continuous massdata of precursor powder in the powder vessel; a control system 400comprising a PID loop and a data processor; and an evaporator assembly140 configured to receive the precursor powder from the powder feederassembly and evaporate the powder.

FIG. 2 shows an exemplary powder vessel assembly 100 equipped with motordriven agitator 210 located inside a powder vessel 215 for housing theprecursor powder 212; a motor driven feed screw 220 set up in horizontalorientation inside a feed screw barrel 230; and an outlet screen 240 atthe end of the feed screw barrel 230. A feed screw 220 may take the formof an actual screw or auger but may in the alternative be comprised ofother mechanical devices known in the art for powder transmission, e.g aconveyor belt, rotating scraper etc. type device. The outlet screen 240may take the form of a mesh screen or vibratory device to aid in breakupand distribution of the powder exiting the feed screw 220 into thereactor (not shown, to be discussed later). In preferred embodiments,the solid precursor material is only in contact with stainless steelsurfaces, preferably electro-polished to a mirror finish to reduceelectrostatic charge that can cause adherence and buildup of powder onthe system's surfaces. Feeder performance may be tailored to achieve atarget feed rate through proper sizing and tuning of the design of theagitator 210, feed screw barrel 230 ID, pitch and thread count of thefeed screw 220 and the size and porosity of the outlet screen 240 for agiven solid material's specific powder flow characteristics.

Feeder performance may be degraded by powder vortexing or “rat-holing,”and/or bridging of the powder material 212 inside of the powder vessel215. Also, settling in the powder vessel may densify the powder andoverload the feed screw 220 drive motor(s) adversely affecting the feedrate oscillations, particularly at low feed screw RPM's. Thus, theagitator 210 aids in reducing these effects, including auto-segregationof powder particles by size within the powder vessel 215 which can causea shift in mass delivery over time. Agitators known in the art includeinternal and external mechanical vibratory devices, rotating barrelswith splines, auger solid mixers and other suitable devices.

In certain embodiments, including typical high-temperaturesuperconductors, the thin film deposition process may require more thanone solid precursor compound or material 212. For more than oneprecursor, each compound may have its own feed system, or a mixture ofmultiple solid precursor compounds may be combined in the same feedsystem. In other embodiments, separate powder vessels may feed a sharedfeed screw assembly. If the solid precursor material is a mixture ofseveral powder components; there is also a danger of componentsseparating (or auto-segregating) in the powder vessel and thus thecomposition of material coming out of the feeder may vary in time. Thus,when precursors are combined in a single powder vessel; auto-segregationby both size and composition can occur. Therefore, in addition tomechanical agitation of the powder vessel, proper pre-conditioning ofthe solid material 212 loaded into the powder vessel such as pre-mixing,grinding, granulating may be conducted to achieve desired feederperformance.

An exemplary load lock assembly 110 for continuous closed systemreloading of the powder vessel 215 is shown in FIG. 3. The assemblyprovides for reloading of the powder vessel 215 of the powder feeder 100without breaking the vacuum environment of a low-pressure vapordeposition system such as used in HTS fabrication. Avoiding interruptionof the thin film deposition process is essential for continuousoperation and production of long length REBCO coated tapes whichrequires that the vacuum conditions be maintained continuously.

Typical operation of the load lock assembly 110 starts with all threevalves shown as valves V1 (310) and V2 (320) and a gate valve 330 closedand the load lock chamber 340 open to the ambient environment. A newload of solid precursor material 212 is added into the load lock chamber340; the chamber is sealed and evacuated by opening valve V1 (310).Pressure in the lock chamber, as monitored by pressure gauge 350 isreduced until it gets lower than the pressure in the feed system powdervessel 215, at which point V1 (310) is closed. Then V2 (320) is openedand a quantity of process gas is drawn through flow control orifice 360from the powder vessel 215 into the load lock chamber 340 until thepressure in both chambers is equalized, at which point the gate valve330 is opened.

Before transferring the precursor material 212 from load lock chamber340 to powder vessel 215, the feeder control system (FIG. 4 discussedbelow) is switched from closed loop control mode to open loop controlmode, in which the feed screw runs at a constant speed. Then theprecursor material 212 is gravity fed from the load lock chamber 340into the powder vessel 215 of the powder feeder assembly 100. The gatevalve 330 and valve V2 320 are closed and closed loop control of thefeeder is reengaged thus completing the reloading cycle. FIG. 11 to bediscussed in greater detail below, graphically plots pertinentcontroller outputs that further illustrate the reloading sequence.

Returning to FIGS. 1 and 2, in preferred embodiments, the powder feedassembly 100 includes a weighing mechanism 120 coupled to the powdervessel 215 and configured to provide continuous mass data of precursorpowder 212 in the powder vessel 215 to a control system 400. Theweighing mechanism 120 may comprise a high precision, high-resolutiongravimetric scales in contact with or affixed to the powder vessel.Hence in this context, the term “coupled” shall mean herein as incontact with, as either affixed to or by mere contact. Such scales knownin the art may be capable of weighing 10's to 100's of grams or severalkilograms to a precision of fractions of a gram, and in certainpreferred embodiments, to one-thousandth of a gram or less withresolution of 1:4,000,000 or greater. It is also readily contemplatedthat the entire powder feed system, or individual components thereofincluding the powder vessel and weighing mechanism, may be in anenclosed housing to minimize drift and error in the measurements.

An exemplary closed loop feeder control system is shown in FIG. 4. Thehigh-resolution weighing mechanism 120 provides an input to a controller400, for example, as a 100 Hz reading 415 processed through a low passfilter 410 from a load cell 420 to generate a 5 Hz weight reading 416. Adata processing algorithm 430 and PID control loop 440 are typicallyimplemented as software code running on a programmable automationcontroller (PAC) 400.

The weight reading input 415 may be in the units of a weight per timefrom which a powder vessel weight change or loss per unit time, e.g.micrograms powder per millisecond, is calculated by the algorithm 430.The scales 120 may generate multiple weight readings per second whichare read and buffered by the PAC. The data 415 as a weight versus timecurve (w(t)) may then be filtered 410 to reduce noise and smoothen thew(t) curve 416. The w(t) curve (415, 416) may then be numericallydifferentiated to calculate the weight loss rate or feed rate curvew′(t) and fit to a curve 418 corresponding to a feed rate. Variousnumerical schemes may be employed by algorithm 430 to treat orprecondition 417 the weight readings, for example, aggregation of 10seconds of weight values on a “Last In First Out” (LIFO) basis, oralternatively for a 10 second, or other suitable duration, on a “Firstin First Out (FIFO) basis. Thus, the data processing algorithm 430 mayconvert a given weight loss 415, 416 curve to a linear or other fittedparametrized curve or formula 418 to generate a calculated feed rate460.

This calculated feed rate 460 may be used as process control variableinput into the PID control loop 440. Output from the PID loop 470 maythen be used as a speed command 540 for the motor driving the feed screw220. Performance of the control system 400 can be optimized by adjustingparameters which are used to filter raw weight readings anddifferentiate the w(t) curve as well as tuning the gains of the PID loop440.

FIG. 5 shows an exemplary plot of the performance of the actual feedrate 530 vs time 520 for a given target or setpoint 510 (left handy-axis) of 120 g/h resulting from a feed screw rate curve 540 (righthand y-axis 550) calculated by the programmable controller. In thisexample, the PID loop 440 of the control system 400 controls the feedscrew rate 540 which is increased over time to compensate for changes inthe powder feed vessel 215 mass over time that occurs over time inbetween recharges of precursor material into the load lock assembly 110.As the powder vessel 215 weight changes over time, the feed screw speed540 is automatically adjusted by the PAC 430 in order to maintain atightly controlled actual feed rate 530 that agrees closely with asetpoint or target 510, in this example, with less than 1% coefficientof variability (CV) actual vs setpoint.

The evaporator assembly 140 is shown in FIG. 6 and may comprise a glassor stainless-steel vertical tube 610 capped at the bottom with a sidehorizontal outlet tube 620. The side walls and the bottom of evaporatormay typically be heated by resistive heaters 625, heat tape or animmersion bath. The precursor powder 212 component or mixture of two ormore precursors, coming out of the feeder feed screw 220 (see FIGS. 1and 2) outlet 340 falls down into the evaporator 610 under the force ofgravity and convection forces from the flow of a carrier gas 630.Carrier gas 630 may be injected into the system at a number of places,for example at the evaporator as shown or up or downstream. Suitablecarrier gasses may include argon, nitrogen or other gas that ispreferably inert. The design and operating parameters of the evaporatorare optimized for consistent continuous operation and full evaporationof the precursors without decomposition while leaving no or minimalresidue in the evaporator.

In other embodiments the calculated feed rate 460 of precursor material212 calculated by the control system 400 may incorporate other and/oradditional process related inputs. For example, as shown in FIG. 7, adeposited layer thickness and growth rate of a thin film can be measuredand monitored as the deposition process is conducted in real time(within the reactor) or near real time (measurement outside thereactor). The reactor 900 may be a Photo-Assisted MOCVD (PAMOCVD)reactor to be discussed in greater detail below and in reference to FIG.9. Techniques known in the art for in situ measurements of thin filmthickness with good experimental error include X-Ray Fluorescence (XRF)among others. In these embodiments, a thin film metric of the depositedlayer on a substrate 720 within reactor 900 is measured and outputtedfrom an XRF device 710 (near real time, outside reactor shown) and isprocessed by the data processing algorithm 430 of control system 400 asa second process variable to check and adjust the conversion of theweight loss curve to the calculated feed rate. In certain otherembodiments, the layer thickness may directly control the feed screw 220speed/rate 540 via the PID loop 440 with or without the weight inputvariable. In other embodiments, elemental composition of the coating maybe measured e.g. by XRF or indirectly by XRD and can serve as an inputto the control system 400. Also, the mass flow of precursor vapor (forexample, as a partial pressure of precursor vapor, or a precursor vaporpressure) can be measured e.g. by gas phase IR optical absorptionspectroscopy or by mass spectroscopy and can serve as an input or as asecondary or auxiliary input. These and other additional inputs mayparticularly apply in a multi vapor source setup (each individualprecursor fed and evaporated separately) where composition of thevapor/coating can potentially be tuned on the fly based on in-processcontrol variables and inputs such as these. In this manner, the feedrate may be directly controlled by the process variable, or the processvariable may serve as a secondary check upon the gravimetric systemdescribed herein.

Performance of the powder feed system is critical for the overall vaporgeneration process and the quality of the deposited thin film coatings,including and particularly high-temperature superconductors (HTS).Variations in precursor feed rate can lead to changes in precursor vaporflow into the deposition zone of a HTS reactor which in turn may causeinconsistent deposited layer thickness, variable and low growth rate andreduced critical current (Ic) performance of a high-temperaturesuperconductor REBCO coating.

The epitaxial REBCO high temperature superconductor (HTS) wire isprocessed in certain preferred embodiments by using Metal OrganicChemical Vapor Deposition (MOCVD), Photo-Assisted Metal Organic ChemicalVapor Deposition (PAMOCVD) or other suitable deposition process known inthe art of superconductor fabrication. The HTS wire or tape typicallyhas a thin film composite architecture, an example of which is shown inFIG. 8. In this example, the architecture includes a substrate 720, atleast one buffer layer (two are shown in this example as 810 and 820),at least one superconducting layer (one is shown in this example as830), and at least one capping or stabilizing layer 840. Other layersare readily contemplated by those skilled in the art and may provideadditional purpose to the basic architecture described herein.

The high-temperature superconducting (HTS) layer 830 is typicallycomprised of HTS materials known in the art capable of generatingsuperconducting behavior at 77K or below which corresponds to theboiling temperature of liquid nitrogen under normal pressure. Suitablematerials may include YBa₂Cu₃O_(7−x) (YBCO) or Bi₂Sr₂CaCu₂O_(8+x)(BSCCO) among others. Other stoichiometries of YBCO are known, includingbut not limited to Y₂Ba₄Cu₇O_(14+x), YBa₂Cu₄O₈ and others, which arealso contemplated by the present disclosure and which are generally andhenceforth will be referred to as YBCO material. In other embodiments,other rare earth (RE) elements may be substituted in place of Y,generally referred to as the family of materials REBa₂Cu₃O_(7−x) (REBCO)where RE may include Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb orLu.

Certain embodiments of REBCO HTS superconductor tapes and wires of thepresent invention may also include nano-sized particles distributedwithin the a-b plane of the superconducting layer of the wire to providehigh Ic at high magnetic fields. Co-pending PCT ApplicationPCT/US19/55745, also assigned to the present Applicant, discloses fluxpinning of HTS materials and is incorporated herein for all purposes. Inthe context discussed herein, said particles within the a-b plane shallmean within the plane that is coplanar to the superconducting layer 830as shown in FIG. 8. In certain preferred embodiments, the orientation ofthe pinning particles is within the a-b plane 850 of the HTS layer 830in contrast to pinning centers aligned with the c-axis 860, whichcorresponds to a direction out of the page in FIG. 8.

Deposition based biaxial texturing of the buffer layer or layers (810,820) may be achieved via Ion Beam Assisted Deposition (IBAD), PulsedLaser Deposition (PLD), or Inclined Substrate Deposition (ISD) or othermethods. The biaxially textured film may have a rock salt (halite) likecrystal structure. The biaxial texturing is necessary for propercrystallographic alignment of the REBCO superconductor layer whendeposited upon the substrate 800 for optimum superconductingperformance. The buffer material may be specified to ensure a desiredlattice mismatch between the buffer (810, 820) and the REBCO HTS layer830 to foster development of the nanoparticles for flux pinning.

For second generation (2G) high temperature superconductors (HTS), theflux pinning force is related to the density, size and dimensionality ofthe defects introduced. In preferred embodiments, thenon-superconducting flux pinning particles are randomly dispersed withinthe superconducting layer. The material composition of thenon-superconducting flux pinning sites can include but are not limitedto RE₂O₃ and BaMO₃. For RE₂O₃, RE may include Y, La, Nd, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb or Lu. In the case of BaMO₃, BaMO₃ nanoparticleformation in REBCO requires the additional element of M where M includesone or more of the following elements: Ti, Zr, Al, Hf, Ir, Sn, Nb, Mo,Ta, Ce, and V.

The size of the non-superconducting flux pinning particles can range upto 100 nm or larger in diameter. RE₂O₃ nanoparticles form within the a-bplanes of the REBCO layer without the need of additional elements beyondthose typically contained in the precursor vapor source for growingREBCO superconducting material. Thus, in preferred embodiments, thenon-superconducting flux pinning particles are co-deposited with thesuperconducting material without introduction of foreign material. It isa further feature of the presently disclosed superconducting wire andfabrication methods that the particles lack a substantial c-axisorientation. The formation of these a-b plane distributed nanoparticlescan be achieved in certain preferred embodiments using a Photo-AssistedMetal Organic Chemical Vapor Deposition (PAMOCVD) process withoutreducing the growth rate as commonly occurs with other growth methodsthat yield preferentially vertically orientated nanoparticles.

FIG. 9 shows an exemplary PAMOCVD reactor 900 and system whereby theapplication of UV and visible light provides the energetic source to thereaction process which may be assisted by thermal radiation to increasethe mobility of the incoming atoms to form the superconducting materialas well as the non-superconducting nanoparticles during the depositionand distribution of both non-superconducting and superconductingmaterial. The UV/visible radiation source 910 is typically enclosedwithin or may be located outside of a low-pressure reaction chamber orvessel 920 maintained at a target pressure by one or more externalvacuum pumps 930. The source 910 may be comprised of one or more lampsemitting a desired wavelength or range of wavelengths toward thesubstrate 720. The lamps may be arranged in the reactor adjacent to orin proximity to the inlet showerhead 940 or may be external to thereactor and focused through a window toward the substrate 720 below theshowerhead which provides injection of precursor 212 from a feed line950 for the precursor starting material. The source 910 is typicallyfocused onto the growth surface of the moving metallic foil substrate720. Such substrate is commonly provided in a reel to reel continuousfeed system with the substrate passing through slits 960 in the walls ofthe reaction vessel 920.

Exemplary YBCO HTS material with non-superconducting flux pining centerscan be produced by MOCVD from a solid precursor feed comprising thefollowing: Y precursor as Yttriumtris(2,2,6,6-tetramethyl-3,5-heptanedionate) (YC₃₃H₅₇O₆, or abbreviatedas Y(THD)₃); Ba precursor as Bariumbis(2,2,6,6-tetramethyl-3,5-heptanedionate) (BaC₂₂H₃₈O₄, or abbreviatedas Ba(THD)₂), and Cu precursor as Copperbis(2,2,6,6-tetramethyl-3,5-heptanedionate) (CuC₂₂H₃₈O₄ or abbreviatedas Cu(THD)₂), where THD is typically an “anion” of2,2,6,6-tetramethyl-3,5-heptanedion (C₁₁H₂₀O₂), and thus THD isC₁₁H₁₉O₂.

The REBCO deposition surface in certain preferred embodiments iscontinually irradiated by the UV/visible radiation flux from theradiation source 910 while the REBCO film is growing with the radiationstriking the tape substrate onto which a REBCO film is being grown at asubstantially normal incident angle as shown in FIG. 9. Normalorientation of radiation yields the highest radiation density at thesurface as any off-normal radiation configuration yields a lowerradiation density. When a radiation source or sources 910 are arrangedin a hemispherical pattern around the inlet showerhead 940, the exposuremay have both perpendicular and non-zero angular radiation striking thesurface.

UV/visible radiation at the surface of the growing film mayenergetically excite surface atoms to enhance their surface mobilitythus allowing for more rapid attainment of their lowest energyconfiguration consequently yielding highly crystalline structure for thegrowing film. It is this highly crystalline structure in the a-b plane(i.e. predominantly within the plane of the substrate) for REBCO thatpromotes high current capacity and high performance. Further, thelocalization of the energy which is promoting growth of the REBCO filmat the growth surface by supplying the energy from above the growingfilm eliminates any thermal lag associated with supply of energy frombelow the tape substrate as in the use of typical heated substratesusceptors.

The UV/visible radiation present at the growth surface of the growingREBCO layer greatly enhances the growth rate of highly textured REBCO.Rates of 1.2 microns/min (μm/min) or higher are possible whilemaintaining the high-performance quality of the REBCO tape. The highgrowth rates are proposed to be due to physico-chemical effectsincluding the mentioned surface diffusion enhancement of the alightingelements forming the REBCO unit cell on the buffer layer surface.Enhancing diffusion of the atoms by UV/Visible radiation as they alightonto the growth surface allows for more rapid movement of atoms to theirlowest energy positions on the surface, and hence higher growth rates.

As stated above, the direct radiation exposure of the growth surfaceresults in REBCO (for example, YBCO) films that can be grown with highcrystalline order and at rates of 1.2 μm/min or higher, and as low as0.01 μm/min, if desired. The REBCO films are grown with a high degree ofcrystalline order or texturing as defined by x-ray diffractionparameters of Δϕ between 2° and 7°, and Δω between 1° and 4° in certainpreferred exemplary embodiments. The performance of the resultingexemplary YBCO wires or tapes as measured by their current carryingcapacity may exceed 500 A/cm-width or higher at 77K. Such performance,and high growth rates allow for industrial production of highperformance REBCO wire with commercially attractive economics.

In preferred embodiments, the flow rates and stoichiometry of thestarting precursor material is highly controlled in order to co-produceRE₂O₃ or BaMO₃ nanoparticles in the REBCO film for flux pinning. Thegrowth rate is adjusted by precise control of precursor flow rates, andsource energy inputs to ensure proper quantity, size and distribution ofnanoparticles. Additionally, the stoichiometry of MOCVD precursor vaporcontributes to the determination of the composition of the secondaryphase non-superconducting particles which act as the pinning centers.The non-superconducting particles of the present invention may incertain embodiments be generated by adding an excess of RE precursor orexcess of Ba and by introducing new M precursor into the vapor flow.Hence, the solid precursor feed system of the present disclosuresignificantly aids the accurate and precisely controlled delivery of HTSprecursors to the deposition zone of the reactor.

A method for manufacturing a high temperature superconductor utilizing asolid precursor feed system is now discussed with regard to FIG. 10. Themethod includes in step 1000 introducing a substrate 800 to a reactor900 provided as part of a system that further includes as step 1010providing an evaporator 140 operatively coupled to a precursor powderfeed assembly 100 that comprises a screw feed device 220 and a precursorpowder vessel 215 coupled to a weighing mechanism 120. In step 1020,precursor powder 212 is loaded into a load lock assembly 110 that isconfigured to supply the precursor powder 212 to the precursor powderfeed assembly 100, wherein the precursor powder 212 is comprised of atleast one component of a high temperature thin film superconductorlayer. Step 1030 includes using a control system 400 to monitor theprecursor powder vessel 215 weight, and in step 1040, using that weightto control the feed screw 220 rate based upon the powder vessel weight215 in order to provide a target precursor powder feed rate to theevaporator 140. In step 1060, the evaporator 140 evaporates theprecursor powder 212 in the evaporator 140 which is transported in step1060 by a carrier gas into the reactor 900 and deposited in step 1070 asa thin film upon the substrate 800 in the reactor 900.

In one exemplary embodiment, Y₂O₃ non-superconducting particles areformed in the YBCO as flux pining centers via PAMOCVD processingutilizing a precursor mixture with 20 atomic % excess Yttrium precursor.The deposition growth rate of HTS material in this example wasapproximately 0.2 μm/min upon a CeO₂ capped IBAD buffered substrate. Inanother embodiment YBCO is deposited with 40 atomic % excess Yttriumprecursor. The deposition growth rate of HTS material in this examplewas approximately 0.25 μm/min upon a LaMnO₃ capped IBAD bufferedsubstrate.

An important performance metric for the HTS wire is to attain highcritical current with the wire containing nanoparticles in the HTS layerfor flux pinning which are distributed along a-b planes in the HTS layerwith no specific vertical or near vertical alignment. Critical currentsgreater than 450 A/cm-width and 0.11 mm total HTS wire thickness can beobtained at 4K and 19 T when the magnetic field is perpendicular to thetape surface (H//c).

The performance of the HTS wire in a magnetic field is also oftencharacterized by a measure commonly referred to as Lift Factor. The LiftFactor is typically defined as the ratio between the critical current at77K, self-field and that at a separate temperature and field such as 4Kand 20 T. Unlike the critical current, which is an absolute value, theLift Factor provides the relative relation of the two values. The wiresof certain exemplary embodiments of the present disclosure havedemonstrated lift at 4K, 20 T (Ic (4K, 20 T)/Ic (77K, self-field)),which corresponds to a Lift Factor of 2 or greater.

The ability to maintain high critical current performance at high growthrate is crucial towards commercial viability of HTS products. Thethickness of the REBCO superconductor layer can be defined by the growthrate of the REBCO multiplied by the deposition time where growth ratescan be 0.2 μm/min, 1.0 μm/min, 1.2 μm/min, 1.5 μm/min and higher whileretaining high flux pinning resulting in critical currents (Ic) above450 A/cm-width at 4K and 20 T and a corresponding engineering criticalcurrent density JE of 40,000 A/cm² or greater, where the engineeringcritical current density JE is defined as the critical current Icdivided by the total cross-sectional area of the HTS wire.

An example of the performance of the powder feed system for HTSfabrication is shown in FIG. 11. In this plot, the HTS wire performancein terms of critical current Ic 1120 at various longitudinal positionsalong the tape (x-axis 1110) is overlaid with the feed system controllervariables. Thus, in this depiction, one can view the magnitude andvariability of a tape output or performance characteristic as a functionof the control system factors operating at the time of deposition forparticular tape locations. The top line 1120 gives the tape criticalcurrent Ic (A/cm-width). Referring also back to the discussion earlierwith regards to FIGS. 3-5; line 1140 of FIG. 11 gives the weight reading415 (as either the actual powder 212 weight or tared powder vessel 215weight) while the load lock assembly undergoes a reloading or rechargeof precursor powder. Line 1150 gives the solid precursor feed rate 460calculated by the control system 400 as a function of the weight reading415 while line 1160 shows the feed screw 220 speed setting 540.

What is claimed is:
 1. A load lock assembly for recharging powder to apowder feed system operated under vacuum, the assembly comprising: apowder containing load lock chamber located separate from a powdervessel of a powder feed system; a plurality of gas control valves incommunication with the load lock chamber; a flow control orificedisposed in line with a gas control valve, a gate valve located inbetween the load lock chamber and the powder vessel and configured fortransferring powder from the load lock chamber to the powder vessel; andwherein the load lock chamber is pressure isolated from the powdervessel and the plurality of gas control valves are configured to providefor addition of powder to the powder vessel while maintaining vacuum inthe powder vessel.
 2. The assembly of claim 1 further comprising atleast one differential pressure gauge configured to a measure a pressurebetween the load lock chamber and the powder vessel.
 3. The assembly ofclaim 1, wherein the flow control orifice is configured to controllablyequalize pressure between the load lock chamber and the powder vesselupon opening of one of the plurality of gas control valves.
 4. Theassembly of claim 1, wherein at least one of the plurality of gascontrol valves and gate valve are in communication with a PIDcontroller.
 5. A precursor powder feed system for deposition of thinfilms, the system comprising: a powder vessel maintained under vacuumfor housing a precursor powder; a load lock assembly located outside thepowder vessel and comprising a pressure isolated chamber for reloadingthe powder vessel, wherein a plurality of gas control valves and a gatevalve pressure isolate the load lock assembly from the powder vessel toprovide for addition of powder precursor while the feed system operatescontinuously, and a flow control orifice to equalize pressure betweenthe load lock assembly and powder vessel; a scale coupled to the powdervessel and configured to provide continuous mass data of precursorpowder in the powder vessel; a mechanically driven powder feeder coupledto the powder vessel and configured to receive powder from the powdervessel; and an evaporator configured to receive the powder from thepowder feeder and evaporate the powder.
 6. The system of claim 5,further comprising a motor driven agitator located within the powdervessel and configured to provide powder mixing and distribution.
 7. Thesystem of claim 5, wherein the evaporator further comprises an outletscreen.
 8. The system of claim 5, wherein the powder feeder is a feedscrew.
 9. The system of claim 5, further configured for injection of aninert carrier gas.
 10. The system of claim 5, further comprising acontrol system having a PID controller with a data processor configuredto control the powder feeder to deliver a target precursor powder feedrate to the evaporator based on the continuous mass data from the scale.11. The system of claim 5, further comprising a control system having aPID controller with a data processor configured to control at least oneof the plurality of gas control valves of the load lock assembly. 12.The system of claim 5, further comprising a control system having a PIDcontroller with a data processor configured to control at least one ofthe plurality of gas control valves of the load lock assembly and thepowder feeder to deliver a target precursor powder feed rate to theevaporator based on the continuous mass data from the scale.
 13. Thesystem of claim 5, wherein the precursor powder is comprised of morethan one thin film component.
 14. A method for manufacturing a hightemperature superconductor, the method comprising: providing a vapordeposition reactor comprising a reactor deposition chamber operatingunder vacuum conditions and configured for depositing a high-temperaturesuperconductor thin film; loading an initial charge of precursor powderto a powder vessel maintained under vacuum and coupled to a scale and amechanically driven powder feeder, wherein the scale is configured toprovide continuous mass data of precursor powder in the powder vessel;coupling a load lock assembly to the powder vessel, wherein the loadlock assembly is located outside the powder vessel and comprises apressure isolated chamber for reloading the powder vessel, wherein aplurality of gas control valves and a gate valve pressure isolate theload lock assembly from the powder vessel, and a flow control orifice toequalize pressure between the load lock assembly and powder vessel;coupling an evaporator in between the powder feeder and the reactordeposition chamber; introducing a substrate to the reactor depositionchamber; monitoring the precursor powder vessel weight in the powdervessel; controlling the powder feeder based upon the powder vesselweight in order to provide a target precursor powder feed rate to theevaporator; evaporating the precursor powder and transporting theevaporated precursor into the reactor for deposition as a thin film uponthe substrate; and recharging the powder vessel with powder precursorvia the load lock assembly while the reactor operates continuously undervacuum.
 15. The method of claim 14, wherein the reactor is aPhoto-Assisted Metal Organic Chemical Vapor Deposition (PAMOCVD)reactor.
 16. The method of claim 14, wherein the powder feeder is a feedscrew.
 17. The method of claim 14, wherein the deposited thin film has agrowth rate of 1.0 μm/min or greater.
 18. The method of claim 14,wherein the superconductor layer further comprises a lift factor at 4K,20 T (Ic (4K, 20 T)/Ic (77K, self-field)) of 2 or greater.
 19. Themethod of claim 14, wherein the superconductor layer further comprises alift factor at 4K, 20 T (Ic (4K, 20 T)/Ic (77K, self-field)) of 3 orgreater.
 20. The method of claim 14, wherein the powder precursor iscomprised of more than one thin film component.